1. Field of the Invention
The present invention relates to a hydraulic drive working vehicle including a variable-displacement type hydraulic pump that is operatively driven by a driving power source, and first and second hydraulic motors that respectively drive first and second wheel positioned on one and the other side in a vehicle lengthwise direction, wherein the hydraulic pump and first and second hydraulic motors are fluidly connected in series one another.
2. Background Art
A hydraulic drive working vehicle in which first and second wheels placed at one side and the other side in a vehicle lengthwise direction are respectively driven by first and second hydraulic motors which are fluidly connected in series to a variable-displacement type hydraulic pump has been widely used in a mowing machine and the like (refer to, for example, U.S. Pat. No. 6,889,793, which will be referred to as a prior art document 1, hereinafter).
More specifically, the prior art document 1 discloses an articulate type hydraulic drive working vehicle including front and rear frames which are placed in front and rear sides of the vehicle and which are coupled to each other in a swinging manner about a pivot shaft along a substantially vertical direction, front and rear wheels which are respectively supported by the front and rear frames, a front-side hydraulic motor for operatively driving the front wheels, a rear-side hydraulic motor for operatively driving the rear wheels, a variable-displacement type hydraulic pump which is fluidly connected in series to the front-side and rear-side hydraulic motors, and a bypass valve capable of selectively realizing a rear-side hydraulic motor driving state for supplying the hydraulic fluid discharged from the hydraulic pump to only the rear-side hydraulic motor by bypassing the front-side hydraulic motor and a front-side/rear-side hydraulic motor driving state for supplying the hydraulic fluid in series to the front-side and rear-side hydraulic motors.
Further, the prior art document 1 discloses an articulate type working vehicle configured so that a length in the vehicle lengthwise direction between the rear wheels and the pivot shaft is smaller than that between the front wheels and the pivot shaft, the articulate type working vehicle having a configuration (hereinafter, referred to as a conventional structure) in which a variable-displacement type hydraulic motor is used as the rear-side hydraulic motor and the capacity of the rear-side hydraulic motor is become small in accordance with a turning radius of the vehicle.
Specifically, in the articulate type working vehicle structured such that the length in the vehicle lengthwise direction between the rear wheels and the pivot shaft is different from that between the front wheels and the pivot shaft, and in a working vehicle structured such that either one of front wheels and rear wheels supported on front and rear portions of a vehicle frame are used as steering wheels, a difference in turning radius between the front wheels and the rear wheels is induced in accordance with the turning radius of the vehicle.
If the capacities of the front-side and rear-side hydraulic motors for driving the front wheels and rear wheels, respectively, are fixed to the same capacity in such a working vehicle in which the difference in turning radius occurs between the front wheels and the rear wheels, a driving speed for one wheels (the rear wheels in the conventional structure) of the front wheels and the rear wheels that have turning radius larger than the other wheels is insufficient during turning movement of the vehicle, thus resulting in a problem that the one wheels are dragged.
The conventional structure described in the prior art document 1 is structured, in consideration of the aforementioned problem, such that the capacity of the rear-side hydraulic motor is made smaller in accordance with the turning angle of the vehicle, in order to overcome the problem of the difference in turning radius caused at a front-side/rear-side hydraulic motors driving state (so-called four-wheel driving state).
As described above, the conventional structure is effective in overcoming the problem of the difference in turning radius at the front-side/rear-side hydraulic motor driving state (so-called four-wheel driving state), but leaves room for improvement with respect to a variable range in which a vehicle traveling speed can be changed.
Specifically, it is desirable to set the variable range of the vehicle traveling speed according to a traveling condition (usage condition), in the working vehicle such as a mowing machine. Namely, it is desirable to change the driving speed (driving torque) for the driving wheels in a lower-speed mode (higher-torque driving mode) when the working vehicle is in a working condition, while it is desirable to change the driving speed (driving torque) for the driving wheels in a high-speed mode (lower-torque driving mode) when the working vehicle is in a traveling condition on loads.
However, with the conventional structure, the variable range of the vehicle traveling speed depends only on the variable range of the capacity of the variable-displacement type hydraulic pump.
Namely, with the conventional structure, the vehicle traveling speed can be varied only within the variable range of the capacity of the hydraulic pump.
Further, as described above, in the articulate type hydraulic drive vehicle described in the prior art document 1, the front wheels and the rear wheels are supported by the corresponding frames, such that the length (hereinafter, referred to as the rear-side length) in the vehicle lengthwise direction between the rear wheels and the pivot shaft is smaller than the length (hereinafter, referred to as the front-side length) in the vehicle lengthwise direction between the front wheels and the pivot shaft.
Although the above configuration in which the rear-side length is made smaller than the front-side length as described above makes it possible to make a short turn, it necessitates, on the other hand, a structure for compensating for the difference in turning radius induced between the rear wheels and the front wheels.
Specifically, in an articulate type front/rear wheels drive vehicle structured such that the rear-side length is smaller than the front-side length, the turning radius of the rear wheels becomes larger than that of the front wheels according to the turning angle of the vehicle.
Accordingly, in assuming that the front and rear wheels have the same outer diameter, it is necessary to make the capacity of the front-side hydraulic motor greater than the capacity of the rear-side hydraulic motor in accordance with the turning angle of the vehicle during the turning movement of the vehicle, while making the capacity of the front-side hydraulic motor equal to that of the rear-side hydraulic motor during the straight traveling movement of the vehicle.
As regards this, the prior art document 1 discloses a link rod that respectively moves in one and the other directions along the axial direction in accordance with the swinging movement of the front frame relative to the rear frame in one direction (for example, in the leftward direction) and the other direction (for example, in the rightward direction) about the pivot shaft, and also discloses a capacity adjustment mechanism with a specific structure which is provided in the variable-displacement type front-side hydraulic motor.
The link rod extends between the rear frame supporting the fixed-displacement type rear-side hydraulic motor and a control shaft of the capacity adjustment mechanism.
More specifically, the rear frame supporting the rear-side hydraulic motor is provided with a swinging connector in which the pivot shaft is inserted, and a horizontal stay is secured to the outer peripheral surface of the swinging connector so as to protrude outwardly in the radial direction with the pivot shaft as a reference.
Further, the link rod has a first end coupled to a free end of the horizontal stay and a second end coupled to a free end of the control arm of the capacity adjustment mechanism.
With this structure, if the front frame supporting the variable-displacement type front-side hydraulic motor is swung in one direction (for example, the leftward direction) and the other direction (for example, the rightward direction) about the pivot shaft relative to the rear frame, the control shaft is rotated in one and the other directions, respectively, through the link rod and the control arm.
And now, a normal capacity adjustment mechanism includes a control shaft supported by a housing of a variable-displacement type hydraulic motor in a rotatable manner around its axis line, and a control arm having a proximal end coupled to an outer end of the control shaft, wherein the control shaft is operatively coupled to a movable swash plate in such a way as to slant the movable swash plate in one and the other directions about a slanting reference line, respectively, in accordance with the rotation of the control shaft itself in one and the other directions about the axis line.
In this case, the slanting movement of the movable swash plate in one direction about the slanting reference line leads to one of the increase and decrease of the capacity of the corresponding hydraulic motor, while the slanting movement of the movable swash plate in the other direction leads to the other one of the increase and decrease of the capacity of the corresponding hydraulic motor. Namely, in the case where the movable swash plate is structured such that its slanting movement in one direction causes the capacity of the corresponding hydraulic motor to increase, the slanting movement of the movable swash plate in the other direction causes the capacity of the corresponding hydraulic motor to decrease.
Accordingly, it is not sufficient to have a configuration in which the control arm of the normal capacity adjustment mechanism is simply coupled through the link rod to the rear frame supporting the fixed-displacement type rear-side hydraulic motor. That is, although the configuration makes it possible to increase the capacity of the variable-displacement type front-side hydraulic motor according to the swinging angle of the front frame relative to the rear frame when the front frame is swung in one direction (for example, the leftward direction) about the pivot shaft, it induces a problem that the capacity of the front-side hydraulic motor is decreased according to the swinging angle of the front frame relative to the rear frame about the pivot shaft when the front frame is swung in the other direction (for example, the leftward direction).
In order to overcome the problem, the capacity adjustment mechanism with the specific structure described in the prior art document 1 includes a control shaft with an eccentric structure.
Specifically, the control shaft includes first to third shaft portions. The first shaft portion is supported by the housing in a rotatable manner about an axis line orthogonal to both the motor shaft and the slanting reference line of the movable swash plate in a state of having an outer end positioned outside the housing of the front-side hydraulic motor and an inner end positioned within the housing. The second shaft portion is positioned within the housing and is eccentric with the first shaft portion. The third shaft portion is eccentric with the second shaft portion so as to be coaxial with the first shaft portion, and is supported in a rotatable manner around its axis line by a port block accommodated in the housing.
Further, the capacity adjustment mechanism includes a coupling shaft provided on an upper end surface of the movable swash plate so as to be substantially in parallel to the first shaft portion, and a plate which couples the second shaft portion to the coupling shaft.
The plate is provided with a round hole in which the coupling shaft is inserted, and an angular hole in which the second shaft portion is inserted.
The angular hole has a length which is substantially equal to the diameter of the second shaft in a first direction parallel to the motor shaft and a length which is larger than the diameter of the second shaft portion in a second direction orthogonal to both the motor shaft and the second shaft portion.
Further, the second shaft portion is positioned at a substantially center portion of the angular hole in the second direction, when the front frame is positioned at a straight travel position with respect to the rear frame.
This structure makes it possible to slide the plate in only one direction in the first direction parallel to the motor shaft in response to the rotation of the first shaft portion of the control shaft in either direction based on the swinging movement of the front frame relative to the rear frame in one or the other direction, thereby slanting the movable swash plate in only a predetermined one direction.
In the articulate type hydraulic drive vehicle described in the prior art document 1, the capacity adjustment mechanism of the variable-displacement type front-side hydraulic motor is needed to have the specific structure in order to compensate for the turning radius difference, as described above, and the port block and the housing of the variable-displacement type hydraulic motor having the specific capacity adjustment mechanism is also needed to have dedicated structures.
Therefore the conventional structure has induced a problem that a common or typical variable-displacement type hydraulic motor cannot be used as the front-side hydraulic motor.
Further, in the articulate type hydraulic drive vehicle configured so that the front wheels include a pair of left and right front wheels and the rear wheels include a pair of left and right rear wheels, there may be a case that a pair of left and right front-side hydraulic motors which are fluidly connected to each other in parallel and a pair of left and right rear-side hydraulic motors which are fluidly connected to each other in parallel are provided, in accordance with a specification.
In such a vehicle in order to compensate for the difference in turning radius, it is necessary to make one of the pair of front-side hydraulic motors or the pair of rear-side hydraulic motors to be of a variable-displacement type and, also, it is necessary to operate the respective capacity adjustment mechanisms for the pair of hydraulic motors of the variable-displacement type in a synchronized manner with each other.
The structure described in the prior art document 1 has a problem that it is difficult to be applied to the articulate type vehicle of this type.